Catalyst deterioration detection system

Abstract

A catalyst deterioration detection system 1 comprises an air-fuel ratio sensor 41, a current detection device 61, a voltage application device 60, a voltage control part 71, an air-fuel ratio control part 72 and a deterioration judging part 73. The air-fuel ratio control part executes fuel cut control, and, after the fuel cut control, executes rich control. The voltage control part, if judging that the air-fuel ratio of the outflowing exhaust gas has reached the stoichiometric air-fuel ratio when setting the applied voltage to a first voltage in a limit current region during the rich control, changes the applied voltage from the first voltage to a second voltage in a limit current region. The deterioration judging part judges the degree of deterioration of the catalyst based on the output current of the air-fuel ratio sensor when the applied voltage is set to the second voltage.

The invention claimed is: 
     
       1. A catalyst deterioration detection system detecting deterioration of a catalyst arranged in an exhaust passage of an internal combustion engine and able to store oxygen, comprising:
 an air-fuel ratio sensor arranged at a downstream side of the catalyst and detecting an air-fuel ratio of outflowing exhaust gas flowing out from the catalyst; 
 a current detector detecting an output current of the air-fuel ratio sensor; and 
 a controller including circuitry configured to:
 apply a voltage to the air-fuel ratio sensor; 
 control the voltage applied to the air-fuel ratio sensor via the voltage applicator; 
 control an air-fuel ratio of inflowing exhaust gas flowing into the catalyst; and 
 judge a degree of deterioration of the catalyst, wherein 
 
 the controller is configured to execute a fuel cut control, in which a supply of fuel to a combustion chamber of the internal combustion engine is stopped, when a predetermined execution condition is satisfied, and, after the fuel cut control, execute a rich control making the air-fuel ratio of the inflowing exhaust gas richer than a stoichiometric air-fuel ratio, 
 the controller is configured to, if judging that the air-fuel ratio of the outflowing exhaust gas has reached the stoichiometric air-fuel ratio when setting the applied voltage to a first voltage in a limit current region during the rich control, change the applied voltage from the first voltage to a second voltage in the limit current region, and 
 judge the degree of deterioration of the catalyst based on the output current of the air-fuel ratio sensor detected by the current detector when the applied voltage is set to the second voltage. 
 
     
     
       2. The catalyst deterioration detection system according to  claim 1 , wherein the first voltage is a voltage whereby the output current of the air-fuel ratio sensor becomes zero when the air-fuel ratio of the outflowing exhaust gas is the stoichiometric air-fuel ratio, and the controller is configured to judge that the air-fuel ratio of the outflowing exhaust gas has reached the stoichiometric air-fuel ratio when the output current of the air-fuel ratio sensor becomes zero. 
     
     
       3. The catalyst deterioration detection system according to  claim 2 , wherein the controller is configured to judge the degree of deterioration of the catalyst based on the output current of the air-fuel ratio sensor detected by the current detector when an elapsed time, from when it is judged during the rich control that the air-fuel ratio of the outflowing exhaust gas has reached the stoichiometric air-fuel ratio, reaches a reference time and the applied voltage is set to the second voltage, and the reference time is 100 ms to 1 second. 
     
     
       4. The catalyst deterioration detection system according to  claim 3 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of unburned gases when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of NOX, the controller is configured to judge that the degree of deterioration of the catalyst is larger the smaller the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       5. The catalyst deterioration detection system according to  claim 3 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of NOX when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of unburned gas, the controller is configured to judge that the degree of deterioration of the catalyst is larger the larger the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       6. The catalyst deterioration detection system according to  claim 2 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of unburned gases when an air-fuel ratio on a upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of NOX, the controller is configured to judge that the degree of deterioration of the catalyst is larger the smaller the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       7. The catalyst deterioration detection system according to  claim 2 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of NOX when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of unburned gas, the controller is configured to judge that the degree of deterioration of the catalyst is larger the larger the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       8. The catalyst deterioration detection system according to  claim 1 , wherein the controller is configured to judge the degree of deterioration of the catalyst based on the output current of the air-fuel ratio sensor detected by the current detector when an elapsed time, from when it is judged during the rich control that the air-fuel ratio of the outflowing exhaust gas has reached the stoichiometric air-fuel ratio, reaches a reference time and the applied voltage is set to the second voltage, and the reference time is 100 ms to 1 second. 
     
     
       9. The catalyst deterioration detection system according to  claim 8 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of unburned gases when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of NOX, the controller is configured to judge that the degree of deterioration of the catalyst is larger the smaller the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       10. The catalyst deterioration detection system according to  claim 8 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of NOX when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of unburned gas, the controller is configured to judge that the degree of deterioration of the catalyst is larger the larger the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       11. The catalyst deterioration detection system according to  claim 1 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of unburned gases when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of NOX, the controller is configured to judge that the degree of deterioration of the catalyst is larger the smaller the output current of the air-fuel ratio sensor detected by the current detector. 
     
     
       12. The catalyst deterioration detection system according to  claim 1 , wherein if the catalyst has a characteristic of deterioration whereby an amount of fluctuation of a rate of purification of NOX when an air-fuel ratio upstream of the catalyst is made near the stoichiometric air-fuel ratio becomes larger than an amount of fluctuation of a rate of purification of unburned gas, the controller is configured to judge that the degree of deterioration of the catalyst is larger the larger the output current of the air-fuel ratio sensor detected by the current detector.